FR2787103A1 - Treatment and management of water purification sludge, used to generate heat, electricity and chemicals in a self-contained system that includes a gasification stage and phosphate recovery - Google Patents

Abstract

Treatment and management of water purification sludge, used to generate heat, electricity and chemicals in a self-contained system that includes a gasification stage and phosphate recovery. The process comprises: (i) extracting (A) of volatiles content at least 70%; (ii) anaerobic digestion of (A) to produce a stabilized/digested sludge (A') of volatiles content at least 60%; (iii) thermal or chemical treatment of (A'); (iv) dehydrating to dry matter content 30-55%; (v) drying to dry matter content at least 90% and (vi) using the product directly as fuel or storing it and then subjecting it to gasification. The coke from gasification is used as fuel or as phosphorus mineral. At least some of the material from step (iv) is used in agriculture. The final product, or the coke produced by gasification, is particularly used as fuel for cement making. The gas produced, a mixture of methane and carbon monoxide (CO), is: (1) burnt and the heat generated used to dry the sludge or (2) the CO is converted to methanol (and this is used as carbon source for biological denitrification of the waste water), while the methane only is used to generate heat and electricity for sludge drying.

Description

The present invention relates to a method for processing and managing

  sludges from urban wastewater treatment plants. It applies more particularly to large treatment plants. At the present time, the biological sludge treatment process originating from purification stations is known, which consists in extracting the sludge in order to subject it to an anaerobic digestion making it possible to obtain stabilized sludge, the latter then being conditioned either by a chemical treatment (treatment with polymers, with iron chloride and lime) or by a heat treatment, the sludge then being subjected to dewatering in particular by

filter presses or centrifuges.

  After this treatment, there are the problems of the disposal and use of the dewatered sludge. In this regard, we can currently envisage several sectors: - agricultural development, by spreading, which poses a large number of problems due in particular to the reluctance of the agricultural professions, to transport costs, in particular in peri-urban areas, to the possible presence of heavy metals in the sludge and the degree of hygienization of this sludge; - an incineration of which the drawbacks are known, in particular the release of carbon dioxide (greenhouse effect), the energy consumption (although this drawback tends to disappear by implementing the autothermicity of the sludge), the release of various pollutants atmospheric and, - a pure and simple landfill, sector which must however

disappear by 2002.

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  If these channels, known and commonly used at present, and in particular agricultural recovery by spreading, can be used for the recycling of sludge from small treatment plants, this is not the case.

  even with regard to sludge from large treatment plants.

  The present invention has therefore set itself the objective of providing a process for the treatment and management of sludge from large capacity treatment plants, this process being designed so as to cause no nuisance, while respecting the standards in force and allowing a valuation

  multiple of sludge: thermal, energetic and chemical.

  Consequently, the present invention relates to a process for the treatment and management of sludge from waste water treatment plants, characterized in that it comprises the following steps: a) extraction of sludge having a volatile matter content of 1 'order of

% or more.

  b) anaerobic digestion of the sludge in order to obtain digested or stabilized sludge,

  with a volatile matter content greater than or equal to 60%.

  c) conditioning of the sludge by thermal or chemical means, d) dewatering, in particular on filter presses or centrifuges of the sludge from the conditioning, in order to obtain a dry matter content of the order of 30 to 55% all or part of this sludge can then be used in

agricultural development.

  e) drying the conditioned sludge, until a dry matter content of 90% or more is obtained, this drying step being preferably carried out so as to obtain granules; f) storage of granules from drying or immediate use as

  fuel, especially in cement works.

  g) recovery and treatment by gasification of the stored granules:

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  - the coke resulting from the gasification treatment which can be used either as a fuel, for example in a cement plant, or as a recyclable phosphorus ore for example in the industries producing fertilizers and, - the gases produced, mixture of methane and monoxide of carbon, which can: - either be burned and produce calories recovered at the sludge drying stage, - or, according to a more complex process, undergo a transformation of carbon monoxide into methanol which will be reused as a source of carbon for the biological denitrification of wastewater, while methane, alone, will be used as fuel in a co-generation of electricity and calories, said calories being able to be used

  in the drying stage.

  The process according to the invention is illustrated schematically by the figure

  1 on which we find the different phases and steps specified above.

  The steps concerning the extraction of the sludge, their anaerobic digestion, their conditioning, as well as their dehydration with a view to an agricultural valorization of the sludges thus treated, are implemented in a conventional and well known manner

  those skilled in the art and under these conditions, these steps will not be described here.

  In this respect, we can refer to the Water Technical Memento, Edition du Cinquantenaire, 1989, edited by DEGREMONT, chap. 24: << Processing of

  urban effluents "and in particular on pages 1309 to 1314 (Volume 2).

  During the sludge drying step with a view to obtaining a dry matter content of approximately 90%, the sludge cakes originating from the dewatering step are dried thermally, by any conventional process and they are transformed in granules. These can be r17- 'It

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  - used as fuel in cement works; - valued in agriculture, as an amendment; - used in landscaping and infrastructure development, - landfilled or, - transformed into gas during the gasification stage. During this gasification step, the granules produced during the previous drying step are transformed into gas in a gasification reactor of known type, operating in a controlled atmosphere. This gasification produces

  coke and a gaseous mixture of methane and carbon monoxide.

  The gasification coke thus obtained can be used as fuel, for example in cement works or as recyclable phosphorus ore, for example in the industries producing fertilizers. We know that the P205 content of phosphate deposits exploited in the world is variable: from 32 to 35% for the richest deposits, 18 to 20% for the average of exploited deposits. The phosphorus content of the coke for the gasification of sludge from the biological treatment of urban wastewater ranges from 10 to 20% of P205, which makes it a particularly interesting deposit, given the ease of operation. (gasification coke is likely to be mixed with the other ores currently exploited) and its residue character, therefore

its minimum cost.

  The gaseous mixture of methane and carbon monoxide can be used: - either to supply a co-generation boiler for the production of electricity and / or heat, the electricity thus produced being returned to the network or used as an energy source in the process steps and the heat being used during the drying carried out after the step of conditioning the sludge by anaerobic digestion;

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  - either to supply a synthesis unit producing methanol from carbon monoxide, methane being used as fuel in

  a subsequent step and the calories produced reused in drying.

  This chemical synthesis unit produces a carbonaceous substrate of methanol type from the gas generated during said gasification step. This substrate, as mentioned above, is used as a carbon source for

  ensure the purification of wastewater by biological denitrification.

  As noted, the process which is the subject of the invention makes it possible to obtain recovery in various forms of all of the sludge produced. The only byproduct, coke from the degasification stage, which contains the

  mineral matter and mineral carbon, being used, as described above

  above, either as fuel, for example in cement factories or, as

phosphorus ore.

  Among the advantages brought by the implementation of the method described above

  above, we can cite in particular the following: - multivaluation of sludge: the process in fact provides various ways of recovering sludge, ranging from the most rustic (recovery by agricultural spreading, landscaping) to the most sophisticated (use of carbon contained in the sludge for denitrification of the water line) and it provides several exit doors for sludge waste; - compliance with all current regulations with regard to sludge and waste; - compliance with the official recommendations relating to a search for the recovery of waste makes it possible to avoid their destruction: as we saw above, the organic matter of the sludge is transformed into electricity and heat and also into a: IiI T - U

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  noble organic product, methanol, reusable in the treatment chain (denitrification); - ecological solution perfectly acceptable by public opinion because it does not cause any nuisance (no dust, no risk of dioxin); - reduction of nuisances, especially olfactory; - possibility of decentralizing the purification, the sludge coming from different water treatment sites being able to be conveyed, to a sludge treatment unit implementing the method of the invention and operating on its own site; obtaining a self-sufficient functioning of the sludge treatment unit, the reuse of waste as raw materials, making it possible to provide energy and reagents to

different stages of the process.

  - Finally, the method according to the invention is particularly advantageous in urban areas where the tonnages of sludge to be spread are

  considerable and limited agricultural land areas.

  It remains to be understood that the present invention is not limited to the embodiments described and / or mentioned above but that it

includes all variants.

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Claims (4)

  1 - Process for the treatment and management of sludge from waste water treatment plants, characterized in that it comprises the following stages: a) extraction of sludge having a volatile matter content of around 70% at less; b) anaerobic digestion of the sludge in order to obtain digested or stabilized sludge having a volatile matter content greater than or equal to 60%; c) conditioning of the sludge by thermal means. or chemical d) dewatering, in particular on filter presses or centrifuges, of the sludge from the conditioning in order to obtain a dry matter content of the order of 30 to 55%, all or part of this sludge which can then be used in agricultural recovery ; e) drying of the conditioned sludge until a dry matter content of 90% or more is obtained; f) storage of the conditioned and dried sludge or immediate use as fuel in a cement plant g) recovery and treatment by gasification of the dried and stored sludge: - coke, resulting from the gasification treatment that can be used either as fuel, in particular in a cement plant; either as a phosphorus ore, and, - the gases produced, consisting of a mixture of methane and carbon monoxide which can: - either be burned to produce calories recovered during the sludge drying stage; 8 2787103   - or undergo a transformation of carbon monoxide into methanol which will be used as a carbon source for the denitrification by biological means of waste water, methane, only being used as fuel in a cogeneration of electricity and calories, said calories can be used   during the drying stage.   2 - Process according to claim 1 characterized in that during the thermal drying step of the sludge, it is transformed into granules.   3 - Method according to one of claims I or 2 characterized in   that the gasification step of the thermally dried sludge is carried out in a controlled atmosphere.   4- Method according to any one of claims   previous characterized in that the carbon monoxide released during the gasification step constitutes the main reagent of a chemical synthesis producing   a carbonaceous substrate of the methanol type.   - Method according to any one of the claims   previous characterized in that the gasification coke is used as   recyclable phosphorus ore in the fertilizer producing industries.